Microwave fiber coating apparatus

ABSTRACT

Apparatus for heating a fiber using microwave energy. The apparatus has a source of microwave energy feeding and applicator. The applicator is configured to concentrate the microwave energy on a chamber holding a fiber. Fibers made or coated using chemical vapor deposition in the apparatus are also disclosed.

This application relates generally to the manufacture of fibers and moreparticularly to the use of microwave heating in a cold-wall reactor forCVD on continuous filaments.

Fibers are used in many applications where high strength and low weightare required. Fibers may be in many forms such as, yarns or continuousfilaments.

For example, fibers, especially continuous filaments, are used in someaircraft parts. In one common use of fibers, the fibers are incorporatedinto a metal or ceramic matrix. The fibers provide strength, stiffnessor toughness to the matrix. Such composites have more useful physicaland mechanical properties than the matrix materials alone.

Such reinforcing filaments are often made by coating a core withmaterials which provide desirable properties, such as strength andstiffness. In some instances, further coatings are applied to make thefiber more compatible with specific matrix materials. Herein, the term"fiber" will be used generically to describe a fiber being made orcoated even though strictly speaking fibers made by coating a core donot exist until after the coating has been applied.

One common way to apply coatings is called chemical vapor deposition (orCVD). In a CVD process, the fiber is passed through a chamber, such as aquartz tube, and heated. The chamber is filled with gases which react atthe heated surface of the fiber and deposits are formed as the result ofthe chemical reaction. In a hot-wall system, the fiber is heated becausethe chamber itself is heated. Hot-wall systems suffer from thedisadvantage that coatings are deposited on the walls of the chamber aswell as on the fiber. The chamber becomes clogged with deposits and mustbe cleaned often.

In a cold-wall system, the fiber itself is heated. Deposition ofmaterials, thus occurs more on the fiber than on the walls of thechamber. Generally, the fiber is heated by passing an electric currentthrough the fiber, causing resistive heating.

To pass current through the fiber, electrical contact is made to thefibers at the ends of the chamber. Typically, the fiber passes through asmall hole in the bottom of a well. The well is filled with mercury,which seals the gasses inside the chamber as well as provides electricalconnection to the fiber.

Though commercial quantities of fibers, particularly continuousfilaments, are coated in existing cold-wall systems, such systems havedrawbacks. One such drawback is that insulative fibers cannot be coatedsince no electrical current can be induced in the fiber. Likewise,insulative coatings cannot be deposited since ounce the coating startsto form there would be no conducting path through the fiber. Anotherdrawback is that highly conductive coatings cannot be deposited either.Once the coating starts to form, the resistance of the fiber drops andeffective resistance heating is not possible. The same result occurs ifa weakly conductive coating is deposited in a thick layer. Theresistance drops as the coating gets thicker.

Also, the use of mercury seals has attendant disadvantages. The mercurymay introduce impurities into the fiber coating.

U.S. Pat. No. 3,754,112 to DeBolt describes one approach to avoid someof the shortcomings of resistive heating. That patent shows RF energyused to induce currents at various places in a fiber. The approachincreased or augmented conventional resistive heating of the fiber. Thisapproach is useful to slightly increase resistive heating as thick,weakly conductive coatings are added. However, such a system, because itstill employs resistive heating, suffers from many of the disadvantagesenumerated above. Further, the system uses relatively high levels of RFradiation. Such radiation creates its own problems, such as interferencewith communication equipment and is cumbersome to shield effectivelywhen used in CVD systems.

SUMMARY OF THE INVENTION

With the foregoing background of the invention in mind, it is an objectof this invention to provide an apparatus for non-resistive heating offibers.

It is also an object to provide an apparatus for heating insulatingfibers.

It is further an object to provide apparatus for coating fibers.

It is yet another object to provide a method for depositing insulatingcoatings on fibers.

The foregoing and other objects are achieved in a Chemical VaporDeposition (CVD) reactor wherein fibers are heated by microwaves. Thereactor comprises a gas filled chamber through which fiber passes. Thechamber is partially disposed in a microwave applicator. In oneembodiment, the microwave applicator comprises a long rectangular boxsupporting propogation of microwaves with a means for concentrating theelectric field around the chamber. In a second embodiment, theapplicator comprises a triangular region with the chamber in front ofthe apex. A plurality of cavities open into the triangular region. Eachcavity has an antenna disposed in it coupling microwave energy into thecavity. Between each cavity and the triangular region a rotating paddleis disposed.

With apparatus according to the invention, gas is introduced into thechamber. The fiber is heated by the microwaves and materials from thegas combine to make solids, which are deposited on the fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by reference to the followingdetailed description and accompanying drawings in which

FIG. 1 is a block diagram of one embodiment of the invention.

FIG. 2 is an isometric view, partially cut away of the waveguideapplicator shown in FIG. 1;

FIG. 3 is a cross section through the waveguide applicator along theline 3--3 in FIG. 2;

FIG. 4 is a block diagram of second embodiment of the invention;

FIG. 5 is an isometric view, partially cut away, of the applicator ofFIG. 4;

FIG. 6 is an isometric view, partially cut away, of the applicator ofFIG. 4 showing additional features of the invention; and

FIG. 7 is a schematic diagram of gas seals according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of a system 10 for depositing coatings onfiber 102. Uncoated fiber is supplied on spool 104. Coated fiber istaken up on spool 106.

Fiber 102 enters a Chemical Vapor Deposition ("CVD") chamber, herequartz tube 112. Tube 112 is of the type conventionally used for CVD onfibers. As in conventional systems, gas supply 114 introduces gases intotube 112. Those gasses are used in the CVD process as described in moredetail below. The unused gasses flow to gas recycler 116 where they areeither processed for reuse or disposal.

Gas is kept inside tube 112 by seals 120. Seals 120 allow fiber 102 topass through tube 112, but allow virtually no gas to escape. Seals 120may be jeweled seals as known in the art. The seals might also be gasseals of the type hereafter defined. Preferably, seals which do notrequire mercury or other liquid metal are used, since according to theinvention no electrical contact is required. Liquid metal residue onfiber 102 may interfere with microwave heating as hereafter described.

Tube 112 runs through applicator 108. Microwave energy is fed toapplicator 108 from microwave generator 110. This microwave energy heatsup fiber 102, causing deposition of a coating on fiber 102. Microwaveenergy not absorbed in applicator 108 is dissipated in load 118 ofconventional type.

The frequency and power provided by microwave generator 110 depends onmany factors and may need to be empirically determined. Here, agenerator capable of producing microwave energy at a frequency of 2.45GHz at power levels adjustable up to around 6 kWatts was used. However,it is predicted that a generator producing maximum power levels up toaround 3 kWatts could also be used.

Mylar sheet 122 separates microwave generator 110 from applicator 108,but allows microwaves to travel into applicator 108. If microwavegenerator 110 contains a cooling fan (not shown), mylar sheet 122prevents the fan from cooling fiber 102 in applicator 108.

Turning now to FIG. 2, details of applicator 108 may be seen. Applicator108 contains relatively long conducting box 202. Box 202 is made from aconventional waveguide, here roughly five feet long. The length is notcritical. The walls of the waveguide have lengths in a ratio of 3:1.Such a waveguide supports propagation in the TE₁₀ mode.

The waveguide has however, been modified. A slot 204 is cut in onesurface of the waveguide. A second slot (308, FIG. 3) is cut in theopposite surface of the box 202.

Slot 204 allows viewing of fiber 102 (FIG. 1) in tube 112. Slot 308(FIG. 3) allows projection 214 to extend into box 202.

Projection 214 is made of a conducting material and is as close to fiber102 as practical. Projection 214 is a means to increase the electricfield density inside tube 112. Increasing the electric field densityincreases the absorption of microwaves by fiber 102 (FIG. 1) andfacilitates heating of fiber 102 (FIG. 1).

Support structures are used to control the shape of box 202. Here, brace222 is used. Several such braces might be needed along the length of box202, but only one brace 222 is shown for clarity.

FIG. 3 shows across section of box 108 taken through brace 222. Asshown, brace 222 is made up of a bracket 302 and a bar 304. Bracket 302and bar 304 are held together by screws 306 as indicated. Tighteningscrews 306 pulls bracket 302 and bar 304 together, thereby slightlychanging the shape of box 108. Thus, screw 306 provide a means to tunebox 108 for propagation of microwaves.

Also, it can be seen in FIG. 3 that projection 214 is a piece of a "T"shaped piece projecting through slot 308. Tightening screws 306 alsodraws projection 214 closer to tube 112.

Returning to FIG. 2, each end of box 202 is connected to a curvedsection 206 or 208. Curved sections 206 and 208 are made from curvedpieces of waveguide as is commercially available. Curved sections 206and 208 are joined to box 202 at flanges 210 and 212, also asconventionally used in microwave equipment.

Curved sections 206 and 208 have been modified to include a hole (notshown) through which tube 112 passes. Each hole (not shown) issurrounded by sleeve 224. The sleeve has a length of approximately onequarter of a wavelength of the microwave energy. Thus, the impedanceinto each hole is very large and very little microwave energy passesthrough the hole.

Slots 220a and 220b in curved sections 206 and 208, respectively, arefor the purpose of tuning applicator 108. It is desirable thatapplicator 108 be tuned to present an impedance to microwave generator110 (FIG. 1) which minimizes reflected power. Pins 226a and 226b can bemoved in slots 220a and 220b to adjust the impedance to a desirablelevel. The desirable level may be determined empirically by moving pins226a and 226b and observing the reflected power by means of a reflectedpower meter (not shown) at microwave generator 110 (FIG. 1). Pins 226aand 226b are simply conducting screws which can be adjusted in lengthextending into the waveguide as well as in position.

Curved section 206 is shown to have soldered to its back wall aprojection 216 which aligns with projection 214. Projection 216 ensuresthat the region in which the electric field is concentrated extends asfar up tube 112 as possible.

It should be noted that projection 216 has a tapered edge 218. Taperededge 218 is tapered at an angle of approximately 15 degrees with respectto the back wall of curved section 206. Tapered edge 218 reducesreflections of microwave energy from projection 216.

A similar arrangement of a projection with a tapered edge aligning withprojection 214 is incorporated in section 208. For clarity, thatprojection is not explicitly shown.

Applicator 108 is constructed using techniques familiar to those skilledin the art of manufacturing microwave equipment. Those techniques arenot explicitly described herein.

The following examples indicate some of the ways the system (FIG. 1) canbe used to coat fibers.

EXAMPLE I

A 142 micron diameter silicon carbide monofilament such as shown in U.S.Pat. No. 4,340,636 was coated with boron nitride (BN) to a thickness of4.3 microns. The waveguide 108 was approximately 25 inches in length.Gas supply 114 provided a mixture of nitrogen (one liter/min),borontrichloride (200 sccm), ammonia (300 sccm) and hydrogen (200 sccm).Microwave generator 110 provided approximately 1800 W to heat the fiberto a temperature between 1450° C. and 1650° C. The fiber was moved at arate of 4 ft./min. Thinner coatings were obtained by moving the fiber atspeeds up to 40 ft/min.

EXAMPLE II

A boron nitride coating was produced similar to Example I except borontrichloride was provided at 22 sccm and ammonia was provided at 50 sccmand nitrogen was provided at 250 sccm. The fiber was moved at a rate of15-17 ft./min. and a BN coating 1.5 microns thick was deposited.

The selection of input gases in examples I and II produced deposits onthe walls of tube 112 which required cleaning after a few hours ofoperation. To avoid these coatings, the reagent gases might be changedto be a mixture of diborane and ammonia. Alternatively, trimethyl boronand ammonia might be used. Also, trimethyl borate and ammonia could beused. Some of these materials are liquids at room temperature andrequire that gas supply 114 be heated to provide a suitable vaporpressure.

EXAMPLE III

Using the same gasses as in example 1, a 2 to 3 mil boron nitridecoating was applied on a 33 micron carbon monofilament.

EXAMPLE IV

Silicon nitride was coated on a silicon carbide monofilament using amixture of silicon tetrachloride and ammonia.

EXAMPLE V

A silicon oxycarbonitride coating on a silicon carbide monofilament canbe produced with gas supply 114 supplying nitrogen bubbled throughhexamethyldisilazane.

EXAMPLE VI

A boron nitride coating can be applied to a core made from amonofilament of alumina doped with iron according to the processconditions in Example I.

Turning now to FIG. 4, an alternative embodiment of the invention isshown. Applicator contains tube 112 with fiber 102 passing through itfrom spool 104 to spool 106. Gas supply 114, gas recycler 116 and seals120 function as previously described to provide a source of reagents forcoating fiber in applicator 408.

Applicator 408 comprises a plurality of cavities 402A . . . 402D. On thefloor of each cavity 402A . . . 402D is an antenna 404A . . . 404D,respectively, which couples microwave energy into each cavity. Antennas404A . . . 404D can be rotated to an optimum position to distributemicrowave energy uniformly into the cavities. The optimum position maybe empirically determined.

Microwave units 406A . . . 406D generate microwaves, such as through theuse of a magnetron (not shown) and couple them to antennas 404A . . .404D, respectively. Microwave units 406A . . . 406D also each contain anexternally controlled motor to rotate antenna 404A . . . 404D,respectively.

Here, cavities 402A . . . 402D, antennas 404A . . . . 404D and microwaveunits 406A . . . 406D were formed by stacking four commerciallyavailable home microwave ovens one on top of the other. These units weremodified for control purposes as herein described.

Control electronics 418 control the generation of microwave radiation bymicrowave units 406A . . . 406D. As in a conventional microwave oven,control electronics 418 can turn each magnetron (not shown) off or on.In addition, control electronics 418 can adjust the level of powerproduced by each magnetron (not shown). This adjustment is accomplishedusing known techniques such as by adjusting the instant heater currentand plate voltage to the magnetrons.

Microwave energy fed into any one of the cavities 402A . . . 402D willcouple into the other three cavities. Without modification, energycoupled into a cavity will be coupled to microwave unit 406A attached tothat cavity. This coupling could present enough energy in the cavity todamage the magnetron (not shown) feeding that cavity. To prevent thisdamage, control electronics 418 control the magnetrons in microwaveunits 406A . . . 406D such that only one is turned on at any given time.

At the mouth of each cavity 402A . . . 402D is disposed a paddle 410A .. . 410D, respectively. Paddles 410A . . . 410D are mounted on shaft414. Motor 412 rotates shaft 414. In operation, paddles 410A . . . 410Ddisperse microwave energy emanating from cavities 402A . . . 402D.

FIG. 4 shows a plurality of tuning vanes 416₁ . . . 416_(n) positionednear tube 112. Each of tuning vanes 416₁ . . . 416_(n) may be adjustedto be closer or further from tube 112. Tuning vanes 416₁ . . . 416_(n)are metal and serve to concentrate the electric field in their vicinity.In operation, certain ones of vanes 416₁ . . . 416_(n) are adjusted tobe closer to tube 112 in regions where fiber 102 is heated to belowaverage temperature while others of vanes 416₁ . . . 416_(n) areadjusted to be further from tube 112 in regions where fiber 102 isheated to above average temperature. In this fashion, the temperature offiber 102 is made more uniform.

Turning to FIG. 5, an isometric view of applicator 408 is shown.Cavities 402A . . . 402D are enclosed in a housing 502. Access can beobtained to housing 502 via door 504 which is hinged via hinges 506.

Door 504 has a triangular section 508. Tuning vanes 416₁ . . . 416_(n)are mounted with set screws 510₁ . . . 510_(n) to be adjustable asdescribed above.

The other side of triangular section 508 contains a plurality of holes512. Holes 512 have a diameter smaller than a wavelength so that nomicrowave radiation passes through holes 512. Holes 512 allowobservation of fiber 102.

Tube 112 is disposed close to triangular section 508 closest to the apexof triangular section 508. This positioning facilitates the focusing ofmicrowave energy toward tube 112.

Where tube 112 exits housing 508, it is surrounded by a one quarterwavelength sleeve 224. This sleeve prevents the release of microwaveradiation from the enclosure.

FIG. 5 shows paddles 410 to have a plurality of blades 514. Blades 514are mounted at an angle of approximately 30 degrees with shaft 414. Thisorientation improves the dispersion of microwaves energy along thelength of tube 112.

Turning to FIG. 6, another feature of the invention may be seen. FIG. 6shows three electric resistance heating elements 602 disposed aroundtube 112. Heating elements 602 and tube 112 are surrounded by a sheet ofinsulation 604. Insulation 604 serves to keep the heat from heatingelements 602 near tube 112.

Insulation 604 is a castable silica ceramic material cast shaped as atube. The tube of insulation is split along its length to allowpositioning around tube 112 and heating elements 602. This material islargely transparent to microwaves and has no effect on microwaves whichheat fiber 102. Heating elements 602 are conventional rod shaped, metalsheathed heating elements. The metal sheath is at ground potential viaconnection to box 502. They also do not impact the microwaves heatingfiber 102 because they are at ground potential.

Heating elements 602 and insulation 604 can heat fiber tube 112 apartfrom any heating caused by microwaves. Heating elements 602 can be usedto heat tube 112 and thereby prevent the condensation of reagents orbyproducts on tube 112.

Turning now to FIG. 7, additional details of seals 120 may be seen.Seals 120 are here referred to as gas seals because no mercury is used.

Each seal 120 comprises two small openings 702 in tube 112. The openingsdefine the boundaries of chamber 704. Gas is supplied to chamber 704from gas supply 710. Here, the gas supplied is nitrogen, but any inert,non-toxic gas could be used. Gas supply 710 keeps the gas in chamber 704at a pressure slightly above atmospheric pressure, and above thepressure of gas in tube 112.

Gas recycler 116 (FIG. 1) keeps the pressure in chamber 706 slightlybelow atmospheric pressure. In operation, reagent gasses from gas supply114 stay in chamber 706 because of the pressure difference betweenchambers 706 and 704. The gas which leaks out of seals 702 is anon-toxic, inert gas. Any gas which passes from chamber 704 to 706 hasno effect on the operation of the applicator since the gas supplied bygas supply 710 is inert.

Having described embodiments of the invention, one of skill in the artmay appreciate that various alternative embodiments could be constructedwith departing from the invention. For example, FIG. 1 shows a two colorpyrometer 124 positioned near applicator 108. Slot 204 (FIG. 2) or holes512 (FIG. 5) allow pyrometer 124 to measure the temperature of fiber102. Based on this measurement, control electronics 126 can vary thepower out of microwave generator 110. In this way, the temperature offiber 102 can be maintained at a constant level. Control electronics andmethods of controlling the power from a microwave generator are known.

As another example of possible variations, the number of paddles 410could be changed. Projection 216 need not be present. If it is not,projection 214 is simply tapered at its ends. Other modifications mightmake the apparatus easier to use. For example, the entire front portionof applicator 108 (FIG. 2) might be hinged to allow easy insertion oftube 112. Also, it was described that vanes 416 were adjusted to providea uniform temperature profile. However, the vanes could also be adjustedto provide a nonuniform temperature profile. Additionally, specificexamples of making or coating monofilaments were disclosed. Theinvention could be employed with yarns, tows, multifilaments, or othertypes of fibrous materials. It is felt, therefore, that the inventionshould be limited only by the appended claims.

What is claimed is:
 1. Apparatus for heating a fiber comprising:a) asource of microwave energy, said microwave energy having an electricfield associated therewith; and said electric field situatedperpendicular to the fiber; b) an applicator having walls defining acavity adapted to have the fiber pass therethrough, said applicatorcoupled to the source of microwave energy; c) means for concentrating amaximum electric field on the fiber comprising an element projectinginto the cavity; and d) means for tuning the electric field in alongitudinal direction along the fiber to yield a uniform electric fieldalong the fiber.
 2. The apparatus of claim 1 wherein the applicatorcomprises a rectangular waveguide having four walls.
 3. The apparatus ofclaim 2 wherein the projecting element comprises a conducting materialprojecting away from one wall of the waveguide.
 4. The apparatus ofclaim 3 adapted to have a fiber pass between the projecting conductingmaterial and a wall of the waveguide.
 5. The apparatus of claim 3wherein the waveguide is adapted to have the fiber pass parallel withits longest side and the projecting conducting material extendssubstantially fully along the longest side of the waveguide.
 6. Theapparatus of claim 1 additionally comprising a tube of materialtransparent to microwaves passing through the applicator.
 7. Theapparatus of claim 6 additionally comprising a conducting sleeve aroundthe tube where the tube enters the applicator, said sleeve having alength of one quarter of a wavelength of the microwave energy.
 8. Theapparatus of claim 6 wherein the applicator comprises a waveguide havinga straight portion and a curved portion and wherein the tube enters theapplicator at the curved portion.
 9. The apparatus of claim 8additionally comprising conducting material projecting away from theperiphery of the waveguide towards the center of the waveguide, saidprojecting conducting material extending along the straight portion ofthe waveguide.
 10. The apparatus of claim 6 additionally comprisingmeans for supplying gas in the tube.
 11. The apparatus of claim 10wherein the tube comprises quartz.
 12. The apparatus of claim 10 whereinthe tube comprises two ends and at least one gas seal is disposed ineach end of the tube.
 13. The apparatus of claim 1 wherein theapplicator presents an impedance to the microwave energy andadditionally comprising means for tuning the impedance of theapplicator.
 14. The apparatus of claim 1 additionally comprising a loadfor absorbing microwave energy exiting the applicator.
 15. The apparatusof claim 1 wherein the source of microwave energy comprises a pluralityof microwave units, each microwave unit having a cavity and a microwaveantenna disposed therein and wherein each cavity opens into theapplicator cavity.
 16. The apparatus of claim 2 wherein the rectangularwaveguide has two walls with a longer dimension and two walls with ashorter dimension and the ratio of the longer dimension to the shorterdimension is 3:1.
 17. The apparatus of claim 15 additionally comprisinga plurality of paddles, wherein each paddle is disposed in one of thecavities.
 18. The apparatus of claim 15 wherein each of the microwaveantennas is fed by a magnetron and further comprising controlelectronics for selectively turning on and off the magnetrons therebyonly one magnetron is on at a time.
 19. The apparatus of claim 17additionally comprising a tube transparent to microwave energy disposedthrough the applicator cavity.
 20. The apparatus of claim 19additionally comprising a conductive projecting element comprising aplurality of conducting projections disposed within the applicatorcavity adjacent to the tube.
 21. The apparatus of claim 20 wherein eachconducting projection is disposed a distance from the tube andadditionally comprising means for adjusting the distance between eachprojection and the tube.
 22. The apparatus of claim 19 additionallycomprising a resistive heating element disposed in the applicatoradjacent to the tube.
 23. The apparatus of claim 1 wherein theapplicator comprises a waveguide having a longest dimension which islong in comparison to a wavelength of the microwave energy.
 24. Theapparatus of claim 23 wherein the longest dimension is in excess of 5times longer than a wavelength.
 25. The apparatus of claim 23 whereinthe longest dimension is in excess of 25 inches.
 26. A microwaveapplicator for heating a fiber having a diameter comprising:a) a waveguide bounded by conductive walls and having a longest dimension andopenings larger than the diameter of the fiber in two of the walls, saidopenings being disposed in walls intersecting the longest dimension; b)a chamber for containing gas, the chamber being disposed within thewalls of the wave guide and extending through the openings, said chamberhaving gas seals through which the fiber passes; and c) a conductiveprojecting member attached to one wall of the wave guide and disposedwithin the wave guide, said projecting member having a longest dimensionparallel with the longest dimension of the wave guide.
 27. The microwaveapplicator of claim 26 wherein the waveguide has a rectangular crosssection.
 28. The microwave applicator of claim 27 wherein the longestdimension of the waveguide exceeds 25 inches.
 29. The microwaveapplicator of claim 28 additionally comprising a source of 2.45 GHzmicrowave energy coupled to the applicator.
 30. The microwave applicatorof claim 26 wherein at least one of the conductive walls has a slottherein and the conductive projecting member is slidably mounted in theslot.